In the last years wireless communication networks, for example for data and/or speech transmissions, became more and more important. One example for such a wireless communication network is a wireless local area network (WLAN), independent of a radio frequency or the like (infrared etc.) used for this network. In such a WLAN, subscriber terminals, such as personal computers, telecommunication equipments, mobile phones, personal digital assistants and the like, are able to perform communications with each other or with corresponding subscriber terminals of other networks (for example wired LANs, WLANs, fixed or mobile telecommunication networks and the like) via corresponding communication protocols.
The general architecture of a WLAN is commonly known and thus described only shortly. The key elements of a WLAN are the subscriber terminals and access points (AP) with which the subscriber terminal communicates over a wireless communication interface, e.g. radio or infrared based. An access point covers a specific area, which is referred to hereinafter as a cell. The size of a cell may vary in dependence of the environment, network operator specifications, number of associated subscribers and the like. The AP is adapted to control communications of the subscriber terminals within this cell, for example, by allocating frequency channels, establishing connections for the subscriber terminals, forwarding data to a destination terminal and the like. A subscriber terminal is normally associated with one access point, which is referred to hereinafter as the serving AP. Furthermore, a distribution network is provided to which the access points are connected. Via the distribution network, communication connections between different APs or external networks (e.g. fixed networks, mobile telecommunications networks such as GSM, UMTS, and the like) can be established for a subscriber terminal. In the case of mobile subscriber terminals, there is a case that a subscriber terminal leaves the cell of its serving AP. In this case, roaming is executed. Roaming means that the subscriber terminal searches an available AP whose connection quality is better than a predetermined threshold or the like and switches the connection to this other available AP, which becomes the serving AP. The decision whether a roaming is to be executed is based, for example, on signal strength measurements and the like. WLANs are implemented according to specific standards. One of these standards is, for example, the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard or its respective extensions such as the IEEE 802.11h standard, which are commonly known to persons skilled in the art.
In the IEEE 802.11 standards, in particular MAC (MAC: Medium Access Control) and PHY (PHYsical layer) protocols are defined. MAC protocol is used, for example, to allow interoperability between compatible physical layers, to reduce a collision probability between different subscriber terminals, and the like. Moreover, the IEEE 802.11 MAC protocol defines beacon frames sent at regular intervals by the access point to allow stations to monitor the presence of the access point. The IEEE 802.11 MAC protocol also gives a set of management frames including Probe Request frames which are sent by a subscriber terminal and are followed by Probe Response frames sent by an available access point, to allow a subscriber terminal to scan actively if there is an access point operating on a certain channel frequency and to show to the subscriber terminal what parameter settings this access point is using. Additionally, a MAC address is provided which is used as an identification element for the respective WLAN elements.
According to the prior art, roaming of a subscriber terminal from one AP to another AP is initialized on the basis of specific configuration settings in the wireless communication network. In accordance with these configuration settings, a subscriber terminal will decide on the basis of communication reception quality measurements whether the recent reception quality is sufficient or not. If not, a commonly known roaming procedure will be initialized by the subscriber terminal.
However, according to this conventional roaming procedure, a load situation in the wireless communication network is not considered in the decision for roaming. In other words, a subscriber terminal could be switched or associated with an AP in which the load situation is such that the communication connection quality is not satisfying. Thus, in conventional wireless communication networks, the load is not balanced between the respective APs.
In document EP 1 156 623 A1, a wireless LAN is described in which the roaming procedure is added by a load balancing function. In order to balance the load within the WLAN, the subscriber terminal receives load information concerning the loading status of the access point from access points. Then, the subscriber terminal may select a communication connection with one of the access points by using a cost function in which the received load information is considered.
However, in specific situations, there might arise a problem in that the load based roaming procedure is not performed in an advantageous manner. In a cellular type of environment the client might be “hesitant” to select the correct cell on the basis of conventional measurement information. Thus, there might be a case that the subscriber terminal has measurement information on hand which causes a very easy or frequent change of the cell, i.e. of the serving AP. In particular in the middle of a transmission this causes undesired pauses to applications, e.g. data transmissions or the like. Typically the roaming in WLAN cells requires some time when changing from serving AP to the neighbor AP. In other words, the effect of such easy roaming is a “stopped” application that continues not until the signaling has been performed in the roaming state. In particular when the subscriber terminal is located in a densely utilized wireless network environment (i.e. a plurality of subscriber terminals is communicating in the respective cells) there could be a so-called “ping-pong” effect. In other words, the decision to change the serving access for the subscriber terminal is made rather frequently and even such that the same two APs are involved in the roaming.